Method of bleaching mechanically-disintegrated wood pulp with hydrosulfite bleach containing an alkali metal citrate



Unite METHOD OF BLEAiIHING MECHANlCALLY-DIS- INTEGRATED WUUD PULP WITH HYDROSUL- FITE BLEACH (ZONTAINING AN ALKALI METAL CITRATE Frederick L. Schucker, Northwoods (Glenside), Pa., as-

signor to Rohm & Haas Company, Philadelphia, Pa., a corporation of Delaware No Drawing. Application July 19, 1955 Serial No. 523,106

Claims. (Cl. 3-104) The present invention relates to an improved process for bleaching wood pulp, and more particularly it concerns an improved process for bleaching mechanically disintegrated wood pulp, commonly referred to as groundwood pulp, involving the use of a water-soluble hydroficial effect. According to U. S. Patent 2,707,144, watersoluble polyphosphates, such as sodium tetraphosphate, enhance the brightness of groundwood pulp obtained by bleaching with hydrosulfites even in the presence of such metal ions.

It. is a principal object of the present invention to provide an improved process for bleaching mechanicallydisintegrated wood pulp wherein the brightness increase is markedly greater than that obtained by prior conventional methods.

Another object of the present invention is to provide an improved method for bleaching mechanically-disintegrated wood pulp involving the use of a water-soluble hydrosulfite wherein the brightness increase realized is markedly greater than that obtained through prior conventional methods involving the use of a hydrosulfite even in the presence of metal ions, such as those of iron, magnesium and copper.

Still another object of the present invention is to provide a simple method for bleaching mechanically-disintegrated wood pulp involving the use of a water-soluble hydrosulfite in conjunction with another agent, whereby the brightness increase obtained is substantially greater than that obtained through the use of a water-soluble hydrosulfite alone.

Further objects and advantages will be apparent from a consideration of the following specification and claims.

It has now been discovered, in accordance with the present invention, that enhanced brightness is also obtained in the hydrosulfite-bleaching of groundwood pulp when the bleaching is efiected at a pH in the range of 4.5 to 6.0 and the bleaching solution contains a small amount of an alkali metal citrate with or without a small amount of an alkali metal tetraborate, and the presence of such metal ions as iron does not interfere with the improvement obtained by the citrates. The action involved when the citrate is employed is not understood. It is not believed to be merely a buffering or a sequestering action since tartrates, acetates, and lactates provide little or no improvement in the hydrosulfite bleaching of groundwood pulp whether or not objectionable metal ions like iron are present. It has also been found that the States Patent ice addition of citric acid alone to sodium hydrosulfite bleaching solutions has little or no efiect on the brightness obtained in bleaching groundwood pulp unless the pH of the solution is practically simultaneously adjusted to the pH range above stated.

Referring to the wood pulp treated in accordance with the present invention, it is, as stated, mechanically-disintegrated wood pulp. There are two types of such wood pulp: (1) ground wood pulp and (2) semi-chemical wood pulp. Ground Wood pulp, as is well-known, is obtained merely by grinding logs to reduce the logs to fibers. Semi-chemical wood pulp is obtained by first reducing the logs to chips, treating the chips briefly with chemicals, such as sodium sulfite and sodium bicarbonate, at elevated temperatures and pressure preliminarily to soften the chips somewhat, and grinding the chips to fibers. Both ground wood pulp and semi-chemical wood pulp are relatively high in lignin content, and, accordingly, present a particularly diflicult problem, from the standpoint of bleaching, as compared to normal chemical pulp.

The pulp treated in accordance with the present invention may be obtained from several different sources. That is to say it may be ground wood pulp or semichemical pulp coming directly from the grinder, or from a point in the pulp-processing procedure remote from the grinder. On the other hand, the pulp coming from the grinder may be first dried and baled, and the bales shipped to a distant point where they are broken up and re-pulped. The pulp treated in accordance with the present invention may be such re-pulped material. The source of the pulp treated in accordance with the present invention is relatively immaterial so long as during the bleaching process it has the proper consistency. Generally, the consistency of the pulp treated in accordance with the present process will be at least about 0.5%,by weight, and the consistency thereof may range up to the point where satisfactory mixing is not feasible, for example up to 10%, or even 15%, by weight, if desired. Preferably, however, the consistency of pulp is between about 2% and about 5% by weight. Generally, both the pulp coming directly from the grinder and the repulped material will have a consistency within these ranges. The consistency of pulp obtained from other points in the pulp-processing treatment may require adjustment, and, if such be the case, the consistency of the pulp can be adjusted to the desired level by simply adding or removing water.

In accordance with the process of the present invention there is added to the stated wood pulp, a water-soluble hydrosulfite as the main bleaching agent. Water-soluble hydrosulfites employed for bleaching wood pulp are well-known and include zinc hydrosulfite, the alkali metal hydrosulfite, ammonium hydrosulfite, and the like. Of the various water-soluble hydrosulfites, sodium hydrosulfite and zinc hydrosulfite, particularly the former, are preferred. The hydrosulfite may be added to the pulp in the solid form or as a solution in water. However, in order to insure intimate mixing of the hydrosulfite with the pulp, it is preferred that the hydrosulfite first be dis solved in water and the resulting solution then mixed with the pulp. The amount of hydrosulfite incorporated in the pulp may vary somewhat depending upon the type of pulp treated, the degree of brightness increase desired, and other factors known to those skilled in the bleaching art. Generally the amount of hydrosulfite mixed with the pulp will be at least about 0.25% by weight, based on the weight of the dry pulp. While amounts of hydrosulfite as high as about 5% or even higher, may be employed, no significant advantage is to be gained by using amounts substantially in excess of about 3% by weight. Preferably, the amount of hydrosulfite employed will be between about 0.5% and about 1.5% by weight based on the dry weight of the pulp.

In one preferred embodiment of the invention, the bleaching is effected by introducing into the groundwood pulp from /2 to 3% of a water-soluble hydrosulfite, such as that of sodium, potassium, or Zinc and from /2% to 3% of an alkali metal citrate, such as that of sodium, potassium, or lithium, the percentages being based on fiber weight in the pulp suspension, or as more commonly expressed on weight of pulp. For most purposes, it is preferred to use from 1 to 2% by Weight (on pulp) of a mixture of the hydrosulfite and citrate in which the weight ratio of hydrosulfite to citrate is from 6:4 to 4:6.

in a modified embodiment, the bleaching is effected by introducing into the groundwood pulp from /2 to 3% (preferably 1 to 2%) of the hydrosulfite; from /4 to3% (preferably to /2%) of the citrate; and from A; to- 1% (preferably to /z%) of an alkali metal tetraborate (the percentage being based on the anhydrous form). zWhen borax (Na l3 O .lH O) is the tetraborate used, A to 2% (preferably /2 to by weight is used whereas the proportion of other hydrated forms (e. g. Na B O .5H O or K B O .8H O) is adjusted according to the degree of hydration.

In both embodiments mentioned, the pH should be maintained in the range of 4.5 to 6.0, and preferably it is Within the narrower range of to 5.7. The attainment of the pH within this range is preferably effected solely by adjustment of the amount of citrate and/or tetraborate. In some cases, it may be necessary to keep the proportion of the citrate or the tetraborate below the upper percentage limits of the broad ranges specified hereinabove in order to keep the pH within the specified pH range.

The bleaching may be eflected at a temperature from 100 F. to 200 F. for a time of about two hours to about Mr hour respectively. Preferably bleaching is effected between 150 to 170 C. for about A to hour. The provision of the stated temperatures will offer no problem to those skilled in the art. In the event fresh ground wood pulp that is the slurry coming directly from the grinder, is being treated, such pulp will generally be at a satisfactory temperature due to the heat generated during the grinding operation.

As in the case with the hydrosulfite, the citrate and/or tetraborate may be added to the Wood pulp as a solid or as an aqueous solution thereof, preferably the latter. The amount of citrate and/or tetraborate employed may vary somewhat depending upon the type of pulp treated and the degree of brightness increase desired.

As stated previously, the citrate and/or tetraborate should be added to the pulp not substantially later than the time when the hydrosulfite is added to the pulp. This means, of course, that the citrate and/ or tetraborate may be added to the pulp at any time before, or at the same time as the hydrosulfite is added thereto, or may even be added directly after the hydrosulfite is added to the pulp. Thus, the term not substantially later when used in connection with the time of addition of the citrate and/or tetraborate to the pulp in relation to the time of addition of the hydrosulfite, means that any delay between the addition of the hydrosulfite to the pulp and the addition of the citrate and/or tetraborate to the pulp will not be such as will permit a substantial amount of the hydrosulfite to be decomposed before the citrate and/or tetraborate is mixed therewith. As stated previously, the citrate and/or tetraborate and the hydrosulfite are preferably mixed substantially simultaneously with the wood pulp. Thus, the hydrosulfite and the citrate and/or tetraborate may be added separately but substantially simultaneously, or the hydrosulfite and the citrate and/or tetraborate may be premixed, for example, in water, and the resulting mixture added to the pulp. This latter means is the preferred way of incorporating and mixing the hydrosulfite and citrate and/ or tetraborate with the wood pulp.

The hydrosulfite and citrate and/or tetraborate, as stated, must be intimately mixed with the wood pulp. This intimate mixture may be obtained through a wide variety of means, such as the use of conventional mixing blades, and the like. The mixing may also be effected at various stages of the paper-making cycle, such as in the beater, using the beater to agitate with a minimum weight against the bed-plates, in a stock chest used for storage, in the stuff-box and so on. The bleaching may be done before or after blending the groundwood pulp or the like with other pulps. In a pulp mill, whether or not it is integrated with paper-making machine or operations, the bleaching may be effected in or immediately following the grinding pit. Following mixing, the hydrosulfite and citrate and/or tetraborate are permitted to remain in contact with the pulp until the desired bleaching has taken place. This contact, after initial mixing, is preferably of quiescent or relatively quiescent type and in confinement to minimize the possibility of entrainment of air. The time of contact may be varied depending upon the amounts of hydrosulfite and of citrate and/ or tetraborate employed, the temperature conditions employed, the type of pulp treated, the degree of brightness desired, as well as the other considerations known to those skilled in the bleaching art.

An advantageous method of mixing the hydrosulfite and citrate and/or tetraborate with the wood pulp, and carrying out the bleaching treatment in general is to continuously feed the bleaching material as a dry solid or in aqueous solution to a continuously-moving stream of the wood pulp. Thereafter, the mixture of bleaching material and pulp is subjected to combined vigorous agitation while confined, as in a conduit, pipe or tower, whereby the solution of bleaching material is intimately mixed with the pulp, and propulsion forcing the mixture of pulp and bleaching material as a continuouslymoving stream, preferably characterized by streamlined plug flow, along a confined, airtight path. The flowing mixture of pulp and bleaching material substantially fills the cross-section of the confined path. The mixture is flowed continuously in the closed circuit until the desired bleaching is obtained. In accordance with a preferred embodiment, the confined, air-tight path may comprise a vertical fluid way, particularly an alternately vertically ascending and vertically descending fluid way, such as a series of upright columns through which the mixture of pulp and bleaching material flows in series. The pulp-consistency, concentration of hydrosulfite and polyphosphate, temperature and pH, are as mentioned above. If a solution of a mixture of hydrosulfite and citrate with or without tetraborate is to be added to the pulp, it may have a concentration of about 1% to 13%, preferably not over 10%. But if a separate solution of the hydrosulfite is to be added, the solutions may have the concentrations mentioned or higher even up to the point of saturation, or slurries may be added. Solutions containing hydrosulfite should be added soon after their make-up, generally in a period of less than two hours after make-up.

After the desired bleaching has been obtained, the pulp suspension may be treated to de-water it, in accordance with the usual practice, or may be made directly into paper. For example, the suspension may be diluted and sent to a bull screen where any chunks of wood not previously removed are removed, following which the suspension may be diluted further, for example to about 0.3 to 0.4% consistency. After the diluted, bleached pulp is run through a fine scren, if desired, the pulp may be de-watered on a wet lap machine, for example, to a consistency of about 25%, and then pressed to a consistency of about 50%.

In the following examples, which are illustrative of the invention, the parts and percentages are by weight unless otherwise noted:

EXAMPLE 1 (a) A dry lap of unbleached groundwood spruce is de-fibered in a heater with a minimum weight on the bed-plate to a consistency of 3% in water containing 18 parts per million of iron.

(b) To separate portions (each of 1660 parts) of the 3% stock obtained in part (a) hereof, there is introduced sodium hydrosulfite in one instance, and a mixture of hydrosulfite with sodium citrate in others, the percentages given in the table being based on dry pulp or fiber weight, and bleached.

(c) Sheets are formed from portions of the unbleached stock of part (a) and from the portions of the ditferently bleached stock after dilution to 0.1% consistency and have a weight of 3 grams for an area of 8 in. by 8 in., and the brightness is determined and converted to units corresponding to those obtained by a General Electric Reflectance Meter. Table I shows the comparative values of brightness and increase of brightness of the bleached sheets over that of the unbleached sheets. The pH is measured at the end of the bleaching.

Table 1 Hydro- Average Increase sulfite, Citrate, pH Brightin percent percent ness of Bright- 4 sheets ness EXAMPLE 2 The procedure of Example 1 is repeated but with two other somewhat darker unbleached groundwood spruce dry lap stock. The results are summarized in Table II.

The procedure of Example 1 is followed except that the water in which the pulp is suspended contains only trace amounts of iron. Table III summarizes the results obtained.

Table III Hydro- Average Increase sulflte, Citrate, pH Brlghtin percent percent ness of Bright- 4 sheets ness EXAMPLE 4 The procedure of Example 1 is followed except that borax is used in addition to sodium citrate-in some in- It will be noted from the above examples that the use of hydrosulfite alone as a bleaching agent increases the brightness about 4.5 to 6 points or units. The use of a small amount of alkali metal citrate in connection with the hydrosulfite increases the brightness 10' to 16 points as compared to the unbleached sheet. The use of the citrate and the tetraborate as well with the hydrosulfite also gives a large increase. This latter procedure allows a small amount of relatively inexpensive borax to replace a large part of the relatively expensive citrate with comparable bleaching effect. This is, therefore, the prepared procedure from the economical viewpoint.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. The method which comprises bleaching a mechanically-disintegrated wood pulp at a consistency of about 0.5% to 10% and a temperature of F. to about 200 F. and a pH of about 4.5 to 6.0 with V2% to 3%, on the weight of pulp, of a water-soluble hydrosulfite bleaching agent and /2% to 3%, on the weight of pulp, of an alkali metal citrate.

2. The method of claim 1 in which sodium hydrosulfite and sodium citrate are used.

3. The method of claim 1 in which sodium hydrosulfite and sodium citrate are used and the ratio of the former to the latter is between 6:4 and 4:6.

4. The method which comprises bleaching a mechanically-disintegrated wood pulp at a consistency of about 0.5% to 10% and a temperature of 100 F. to about 200 F. and a pH of about 4.5 to 6.0 with /2 to 3%, on the weight of pulp, of a water-soluble hydrosulfite bleaching agent and A to 3%, on the weight of pulp, of an alkali metal citrate, and /8 to 1%, on pulp weight, of an alkali metal tetraborate.

5. The method which comprises bleaching a mechanically disintegrated wood pulp at a consistency of about 0.5 to 10% and a temperature of 100 F. to about 200 F. and a pH of about 4.5 to 6.0 with 1 to 2%, on the Weight of pulp, of a water-soluble hydrosulfite bleaching agent and A to /2%, on the weight of pulp, of an alkali metal citrate, and A; to 1%, on pulp weight, of an alkali metal tetraborate.

6. A method as defined in claim 5 in which sodium hydrosulfite, sodium citrate, and sodium tetraborate are used.

7. In the bleaching of mechanically-disintegrated wood pulp with a water-soluble hydrosulfite bleaching agent in an amount of at least 0.25% on the weight of dry pulp, the improvement which comprises mixing with said wood pulp having a consistency of at least 0.5 and a temperature between 100 F. and about 200 F., at a time not substantially later than that at which the hydrosulfite is mixed with the pulp, to 3% of an alkali metal citrate, based on the weight of dry pulp, the amount of citrate being adjusted to provide a pH of the mixture between 4.5 and 6, and the hydrosulfite and citrate are in admixture with said wood pulp during bleaching.

8. In the bleaching of mechemically-disintegrated wood pulp wherein a water-soluble hydrosulfite bleaching agent is mixed with the pulp in an amount of ,at least 0.25% on the weight of dry pulp, the improvement which comprises mixing with the wood pulp having a consistency of at least 0.5% and a temperature between 100 F. and about 200 F., substantially simultaneously with the hydrosulfite, A to 3% of an alkali metal citrate based on the weight of dry pulp, the amount of citrate being adjusted to provide a pH of the mixture between 4.5 and 6.

9. In the bleaching of mechanically-disintegrated wood pulp with a water-soluble hydrosulfite bleaching agent in an amount of at least 0.25% on the weight of dry pulp, the improvement which comprises mixing with said wood pulp having a consistency of at least 0.5% and a temperature between 100 F. and about 200 F., at a time not substantially later than that at which the hydrosulfite is mixed with the pulp, about /2% to 3%, on the weight of the pulp, of an alkali metal citrate, the amount of citrate being adjusted to provide a pH of the mixture between 4.5 and 6, and the hydrosulfite and citrate are in admixture with said wood pulp during bleaching.

10. In the bleaching of mechanically-disintegrated wood pulp with a water-soluble hydrosulfite bleaching agent in an amount of at least 0.25% on the weight of dry pulp, the improvement which comprises mixing with said wood pulp having a consistency of at least 0.5% and a temperature between 100 F. and about 200 F., at a time not substantially later than that at which the hydrosulfite is mixed with the pulp, to 3% of an alkali metal citrate and to 1% of an alkali metal tetraborate, both based on pulp Weight, the amount of citrate and tetraborate being adjusted to provide a pH of the mixture between 4.5 and 6, and the hydrosulfite, citrate and tetraborate are in admixture with said wood pulp during bleaching.

References Cited in the file of this patent UNITED STATES PATENTS 2,077,983 Block Apr. 20, 1937 2,236,970 Goldfarb Apr. 1, 1941 2,395,311 Woodhouse Feb. 19, 1946 2,707,144 Sparrow et al Apr. 26, 1955 2,707,145 Sparrow et a1 Apr. 26, 1955 

4. THE METHOD WHICH COMPRISES BLEACHING A MECHANCALLY-DISINTEGRATED WOOD PULP AT A CONSISTENCY OF ABOUT 0.5% TO 10% AND A TEMPERATURE OF 100*F. TO ABOUT 200*F. AND A PH OF ABOUT 4.5 TO 6.0 WITH 1/2 TO 3%, ON THE WEIGHT OF PULP, OF A WATER SOLUBLE HYDROSULFITE BLEACHING AGENT AND 1/4 TO 3%, ON THE WEIGHT OF PULP, OF AN ALKALI METAL CITRATE, AND 1/8 TO 1%, ON THE PULP WEIGHT, OF AN ALKALI METAL TETRABORATE. 